The exhaust gas back pressure after the turbo charger(s) depends on the total pressure drop in the exhaust gas piping system.
The components, exhaust gas boiler, silencer, and spark arrester, if fitted, usually contribute with a major part of the dynamic pressure drop through the entire exhaust gas piping system.
The components mentioned are to be specified so that the sum of the dynamic pressure drop through the different components should, if possible, approach 200 mm WC at an exhaust gas flow volume corresponding to the specified MCR at tropical ambient conditions. Then there will be a pressure drop of 100 mm WC for distribution among the remaining piping system.
Fig. 15.05.01 shows some guidelines regarding resistance coefficients and back pressure loss calculations which can be used, if the maker’s data for back pressure is not available at an early stage of the project.
The pressure loss calculations have to be based on the actual exhaust gas amount and temperature valid for specified MCR. Some general formulas and definitions are given in the following.
Exhaust gas data
M: exhaust gas amount at specified MCR in kg/sec. T: exhaust gas temperature at specified MCR in °C
Please note that the actual exhaust gas temperature is different before and after the boiler. The exhaust gas data valid after the turbocharger may be found in Chapter 6.
Exhaust gas velocity (v)
In a pipe with diameter D the exhaust gas velocity is:
v = Mρ x π x4D2in m/s
Pressure losses in pipes (∆p)
For a pipe element, like a bend etc., with the resistance coefficient ζ, the corresponding pressure loss is:
∆p = ζ x ½ ρ v2 x 9.811 in mm WC
where the expression after ζ is the dynamic pressure of the flow in the pipe.
The friction losses in the straight pipes may, as a guidance, be estimated as :
1 mm WC per 1 diameter length
whereas the positive influence of the up draught in the vertical pipe is normally negligible.
Pressure losses across components (∆p)
The pressure loss ∆p across silencer, exhaust gas boiler, spark arrester, rain water trap, etc., to be measured/ stated as shown in Fig. 15.05.01 (at specified MCR) is normally given by the relevant manufacturer.
Total back pressure (∆pM)
The total back pressure, measured/stated as the static pressure in the pipe after the turbocharger, is then:
∆pM = Σ ∆p
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